This invention relates to the art of induction heating and, more particularly, to a method and inductor for inductively heating and annealing work hardened portions of a structural beam.
It is well known that structural beam members are often produced by cold forming initially flat metal plate to a desired configuration in which the beam has at least two planar walls with a relatively sharp corner therebetween. One such configuration is a hollow beam which is square in cross-section, and the present invention is described in detail herein in connection with such a beam configuration. However, it will be appreciated that the invention is applicable to other beam configurations.
In cold forming a hollow beam from initially flat sheet metal, the sheet is bent to provide the beam with opposed pairs of planar wall portions having relatively sharp corners between adjacent ones of the wall portions. The longitudinally extending outer side edges of the sheet generally abut centrally of one of the side portions and are welded together to complete the beam. The cold working process results in work hardening of the metal in the corner areas of the beam, whereby the corner areas are much harder than the wall portions of the beam. Therefore, when the beam is loaded, such as in compression for example, the load is primarily borne by the corners of the beam as opposed to being uniformly distributed through the corners and wall portions of the beam. Accordingly, it becomes desirable to reduce the hardness in the corner portions without changing the hardness in the wall portions and so that the corner portions and wall portions are of substantially the same hardness.